Such sources, which are called cyclotron resonance masers, for example gyrotrons, have successfully been used for plasma build-up, profile control and resonance heating in the field of thermonuclear fusion research at power levels in the MW-range. The oscillation of these sources is in axially symmetric modes TE.sub.On or in high asymmetric modes (TE.sub.mn) with m&gt;&gt;1.
As future fusion devices are planned to be larger and to operate at higher magnetic fields, sources must be developed for higher frequencies (above 100 GHz) and higher microwave power. For these applications, it would be desirable to dispose of an axisymmetric narrow pencil-like beam with well defined linear polarization, as for example a TEM.sub.OO Gaussian beam. In fact, this kind of beam is not only required for efficient electron cyclotron resonant heating of a plasma, but also for low-loss high power millimeter wave transmission.
From IEEE Transactions Microw. Th. Techn., Vol. NTT-26 No. 5, 1978, page 332 to 334, a mode transducing antenna is known, having a corrugation by which a TE.sub.On mode is converted into a linearily polarized beam. However, the microwave radiation issuing from such an antenna still presents considerable sidelobes which reduce the energy concentration in the main lobe and increase the losses of the system.
The main object of the present invention is thus to reduce the energy dissipated in the sidelobes and to provide an axisymmetric, narrow, Gaussian-like main lobe.
According to a particular application of the present invention, the antenna system provides for an output radiation of the linear TEM.sub.OO -mode type.
These objects are achieved according to the invention by the antenna system as defined in the attached claims.
The invention will now be described in greater detail by means of several embodiments and in relation to the attached drawings.